This invention relates to a reference voltage generating circuit, particularly to the reference voltage generating circuit wherein a temperature characteristic of an output voltage is set as desired.
In the past, as a reference voltage generating circuit has been used a circuit shown in FIG. 1 comprising a resistor r.sub.L having a resistance value R.sub.L, a source of constant current or constant current source 1 supplying a constant current I.sub.0, and an emitter follower constituted by a transistor Q.sub.1 and a resistor r.sub.EF. In such a reference voltage generating circuit, the emitter-collector voltage of transistor Q.sub.1 is used as a reference voltage output and the temperature characteristic of this voltage V.sub.R is set by controlling the temperature characteristic of the current I.sub.0 of the constant current source 1.
FIG. 2 shows another example of the prior art reference voltage generating circuit. As shown in FIG. 2, the constant current source 1 is generally constituted by a transistor Q.sub.2 and a resistor r.sub.E having a resistance value of R.sub.E and the current value I.sub.0 is controlled by selecting a suitable temperature characteristic for an output voltage V.sub.CS of a source of drive voltage or the driving voltage source 2 which is often commonly used for driving a plurality of gate circuits or the like other than the reference voltage generating circuit. A transistor Q.sub.4 and a resistor r'.sub.E having a resistance value R'.sub.E supplies a constant current I.sub.1 to any circuit network G and the driving voltage source 2 is commonly used for the reference voltage generating circuit and the circuit network G.
In FIG. 2, where the base-emitter forward voltage of the transistor Q.sub.3 denoted by V.sub.F, the current I.sub.0 of the source of constant current is expressed by EQU I.sub.0 .apprxeq.(V.sub.CS -V.sub.F)/R.sub.E
Hence the reference voltage V.sub.R is expressed by ##EQU1##
In this equation, if we assume that a ratio R.sub.L /R.sub.E is constant irrespective of a temperature variation, the temperature characteristic of the reference voltage V.sub.R is given by ##EQU2## Since dV.sub.F /dT can be considered as a physical parameter of the transistor, in order to make dV.sub.R /dT to a desired value, the temperature characteristic of the drive voltage V.sub.CS should be determined to satisfy the following equation ##EQU3##
In the same manner, the desired temperature characteristic dI.sub.1 /dT of current I.sub.1 of another constant current source which also uses the driving voltage source 2 is expressed by ##EQU4## In this equation, since dV.sub.F /dT and dR'.sub.E /dT are considered as the physical parameters of transistor Q.sub.4 and resistor r'.sub.E respectively, where the value of dV.sub.CS /dT satisfying dI.sub.1 /dT does not coincide with dV.sub.cs /dT satisfying equation (1), either one of the constant current sources should be driven by an independent driving voltage source for setting different ratio V.sub.CS dT. In other words, it is impossible to provide any temperature characteristic for only the reference voltage generating circuit. This not only causes increase in the occupation area or volume and power consumption of semiconductor devices but also increases the number of driving voltage sources which are required to be designed precisely, thereby increasing the number of steps of design.